The present invention relates to a discharge screw for discharging loose materials from a moving hearth, such as annular and rotary hearths for reheat furnaces, straight horizontal or inclined conveyor belts, or the like.
In particular, the present description will refer by way of non-restrictive example to a discharge screw used in a reheat furnace having an annular rotary hearth for treating loose materials, such as minerals in pellet form subjected to an oxidation-reduction heat treatment.
As is known, these reheat furnaces comprise a toroidal chamber with an annular hearth that rotates with respect to the rest of the chamber, and two side walls and an upper wall, with a plurality of burners arranged on these walls. The pellets are introduced onto the surface of this annular hearth and must be thermally treated by the heat supplied by the burners: the pellets, after performing approximately one revolution of the furnace under the action of the said burners, arrive upstream of a screw which is positioned radially above the hearth. The rotation of the said screw causes the pellets to be discharged onto one or both sides of the said hearth. For discharge onto both sides of the hearth, one possibility is to use a rotary screw consisting of a cylinder comprising on its lateral surface a first series of helicoids beginning in an area close to its outermost end with respect to the furnace and a second series of helicoids beginning at its innermost end with respect to the furnace. The inclination of the helicoids of the first series is essentially symmetrical to the inclination of the helicoids of the second series. These two series of helicoids meet approximately in a transverse plane near the centre of the cylinder and on this plane the said helicoids come together exactly.
There are drawbacks to these screws with helicoids whose inward ends meet and come together in a transverse plane with respect to the cylinder: during treatment in the furnace, the pellets occupy virtually the whole of the surface of the rotary hearth and, as they arrive upstream of the discharge screw, those positioned beneath the meeting point of the helicoids are not discharged efficiently, causing an accumulation of material which progressively builds up, because the material is being continually fed in at the loading area downstream of the screw.
The object of the present invention is to provide a discharge screw for a moving hearth, and in particular for a rotary hearth in a reheat furnace, that will overcome the abovementioned drawbacks of known screws.
This object is achieved by the present invention in the form of a discharge screw for a moving hearth, and in particular for an annular rotary hearth of a reheat furnace for the heat treatment of loose materials, comprising a cylinder arranged above and substantially radially with respect to this annular hearth and turned by suitable actuators so as to discharge the materials from the edges of this annular hearth; this cylinder comprises on its lateral surface a first series of helicoids beginning in an area close to its outermost end with respect to the furnace and a second series of helicoids beginning at its innermost end with respect to the furnace; and the inclination of the helicoids of this first series is essentially symmetrical to the inclination of the helicoids of this second series; in this screw, each helicoid of one series comprises an inward end that meets the inward end of a corres-ponding helicoid of the other series in such a way that at at least one of the abovementioned ends a free final section is left for the removal of loose materials from an area underneath it on the annular hearth.